Method and device for controlling the temperature of exhaust gas from an internal-combustion engine flowing through a means of treating the pollutants contained in this gas

ABSTRACT

The present invention relates to a method for controlling the temperature of exhaust gas circulating in an exhaust line ( 20 ) of an internal-combustion engine ( 10 ), said line comprising means ( 30 ) of treating the pollutants contained in this gas and heat exchange means ( 32 ) for cooling or heating said exhaust gas flowing through these pollutant treatment means. 
     According to the invention, the method consists in using evaporator ( 38 ) of a fluid circulation closed circuit ( 34 ) as heat exchange means ( 32 ).

FIELD OF THE INVENTION

The present invention relates to a method and to a device forcontrolling the temperature of exhaust gas from an internal-combustionengine, notably of Diesel type, flowing through a means of treating thepollutants contained in this gas.

BACKGROUND OF THE INVENTION

In order to comply with environmental standards and to meet theseverization of these standards, such as the standards known as EURO VI,these pollutants, in particular the nitrogen oxides (NO and NO₂), morecommonly referred to as NOx, contained in this exhaust gas, have to betreated prior to discharging it into the atmosphere.

As it is generally known, pollutant treatment devices are thereforeinstalled on the exhaust line of vehicles.

This exhaust line comprises, from the exhaust manifold and in thedirection of circulation of the exhaust gas along the line, a three-waycatalyst, referred to as triple-effect catalyst, whose purpose is totreat, through oxidation, the unburnt hydrocarbons (HC) and the carbonmonoxide (CO) contained in the exhaust gas, and an SCR (SelectiveCatalytic Reduction) catalyst for treating the NOx.

This SCR catalyst allows to selectively reduce the NOx to nitrogenthrough the action of a reducing agent. This agent, which is generallyinjected upstream from the catalyst, can be a hydrocarbon, hydrogen,carbon monoxide, ammonia or a compound generating ammonia throughdecomposition, such as urea.

The problem that arises with such a device is that the SCR catalyst hasan operating temperature ranging from around 300° C. to around 500° C.

Now, it is not always possible to finely control the exhaust gastemperature because it greatly depends on the engine operating point.Thus, for high engine loads notably, the exhaust gas temperature canwidely exceed 500° C. This high temperature, on the one hand, does notallow the catalyst to fulfil its NOx reduction function and, on theother hand, it can lead to a degradation of the constituent material ofthis catalyst and/or of the catalytic phases it comprises.

It is already known to arrange heat exchange means on the exhaust lineand upstream from the SCR catalyst in order to cool the exhaust gasbefore it enters this catalyst.

A cooler through which the engine cooling fluid flows is thereforearranged on the exhaust line. This cooler allows to absorb the heatcontained in the exhaust gas and thus to control the temperature of thisgas so that it does not exceed the upper value of the operatingtemperature range of this catalyst.

This however notably involves the drawback of generating an increase inthe temperature of the cooling fluid that is heated by the exhaust gas.Since this fluid is also used for cooling the engine and otheraccessories of this engine through an intercooler, it is thereforenecessary to increase the capacity of this intercooler. In case oflimited space for the intercooler, it is necessary to provide additionalintercoolers so as to bring this cooling fluid to a requiredtemperature. This leads to a quite significant cost increase and tocomplexity of the engine general cooling circuit.

Furthermore, when the temperature of the gas is lower than that requiredto provide smooth operation of the catalyst, notably when starting theengine or after stopping this engine, the exhaust gas containing NOx isnot treated by the catalyst and it is discharged into the atmospherewith the pollutants it contains.

The present invention aims to overcome the aforementioned drawbacks bymeans of a simple and economical device allowing to have exhaust gas atthe required temperature to provide depollution treatment of this gas,whatever the operating conditions of the internal-combustion engine.

SUMMARY OF THE INVENTION

The present invention therefore relates to a method for controlling thetemperature of exhaust gas circulating in an exhaust line of aninternal-combustion engine, said line comprising means of treating thepollutants contained in this gas and heat exchange means for cooling orheating said exhaust gas flowing through these pollutant treatmentmeans.

The method can consist in feeding a hot fluid into the evaporator sothat it heats the exhaust gas.

The method can consist in using a hot fluid contained in a fluid storagemeans connected to the circuit.

The method can consist in connecting the storage means to the evaporatorby means of a bypass line.

The invention also relates to a device for controlling the temperatureof exhaust gas circulating in an exhaust line of an internal-combustionengine, said line comprising means of treating the pollutants containedin this gas and reversible heat exchange means for cooling or heatingsaid exhaust gas flowing through these pollutant treatment means,characterized in that the reversible heat exchange means are anevaporator of a fluid circulation closed circuit.

The circuit can comprise a fluid storage tank.

The tank can be a thermally insulated tank.

The tank can comprise fluid heating means.

The circuit can comprise a bypass line for allowing the fluid from thetank into the evaporator.

The bypass line can comprise a throttling means.

The reversible heat exchange means can comprise at least one thermopile.

The thermopile can be connected to electric accumulators for supplyingpower thereto when heating the exhaust gas.

The pollutant treatment means can comprise a selective catalyticreduction catalyst.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be clear fromreading the description given hereafter by way of non limitativeexample, with reference to the accompanying figures wherein:

FIG. 1 is a diagram showing a device for controlling the temperature ofexhaust gas from an internal-combustion engine according to theinvention,

FIG. 2 is a more detailed view of a part of the device of FIG. 1, and

FIG. 3 is another diagram illustrating a variant of the device as shownin FIG. 1.

DETAILED DESCRIPTION

In connection with FIG. 1, internal-combustion engine 10, notably ofDiesel type, comprises at least one cylinder 12, an intake manifold 14and an exhaust manifold 16 allowing to collect the exhaust gas resultingfrom the combustion of a fuel mixture in the cylinders prior to sendingit to inlet 18 of an exhaust line 20.

In FIG. 1, the exhaust line carries, in the direction of circulation ofthe exhaust gas from inlet 18 of this line to its outlet (symbolized byarrow 22), a three-way oxidation catalyst 24 arranged as close aspossible to exhaust gas inlet 18, followed by an injector 26 for areducing agent, urea here, arranged opposite inlet face 28 of an SCRcatalyst 30. This line also carries an exhaust gas temperature detector31 housed opposite and close to inlet face 28 of the SCR catalyst. Thisdetector allows, in combination with the calculator anyinternal-combustion engine is usually provided with, to know thetemperature of the exhaust gas entering this catalyst.

As more visible in FIG. 1, a reversible heat exchanger 32 is providedbetween the two catalysts to control the temperature of the exhaust gasreaching inlet face 28 of the SCR catalyst. Advantageously, thisexchanger is arranged downstream from the three-way catalyst andupstream from the temperature detector.

This exchanger allows to cool the gas or to heat it so that itstemperature is in the usual operating range of the SCR catalyst,generally between around 300° C. and around 500° C.

More precisely, in connection with FIG. 2, this exchanger is part of afluid circulation closed circuit 34, more particularly of Rankine cycletype.

This circuit comprises a means 36 intended for circulation andcompression of a working fluid, water here, circulating clockwise inthis circuit (arrows A). This means, referred to as compressor, allowsto compress this water and it is advantageously driven in rotation byany known means such as an electric motor (not shown).

This circuit comprises, after the compressor, a heat exchange means 38,an evaporator here, traversed by the compressed water that leaves it incompressed steam form.

This evaporator is arranged on portion 40 of exhaust line 20 containedbetween the two catalysts 24 and 30, preferably upstream from injector26 and detector 31, so as to be able to cool the exhaust gas circulatingin this portion or to heat it.

The person skilled in the art may consider any possible configurationfor arranging this evaporator in direct or indirect connection with thisexhaust line portion so as to provide the best heat exchange possiblewith the exhaust gas.

After this evaporator, the circuit also comprises an expander 42receiving from this evaporator the high-pressure compressed steam thatflows therefrom in form of low-pressure expanded steam.

This expander can be, by way of example, an expansion turbine whoserotor is driven in rotation by the steam. This rotor is advantageouslyconnected to a device for converting the mechanical energy recovered toanother energy, such as an electric generator for example.

The circuit also comprises a cooling exchanger 44 or condenser receivingthe expanded low-pressure steam from the expander that is converted, atthe outlet of this condenser, to water in liquid form. This condenseris, in the example of FIG. 1, swept by a cooling fluid that isadvantageously outside air at ambient temperature.

Fluid circulation lines allow to successively connect the variouselements of this circuit so that the working fluid, in liquid or vapourform, circulates in the direction shown by the arrows. More precisely,this circuit comprises a line 46 between the compressor and theevaporator, a line 48 between the evaporator and the expander, a line 50between the expander and the condenser, and a line 52 between thecondenser and the compressor.

Furthermore, this circuit comprises a bypass line 54 that starts on line48 between the evaporator and the expander, and ends on line 52 betweenthe condenser and the compressor. Besides, an advantageously thermallyinsulated tank 56 is connected to line 48 by a connecting line 58 thatends on the portion of line 48 contained between the evaporator and thestarting point of bypass line 54. This bypass line and connecting line58 carry each a throttling means allowing to control the fluidcirculation in these lines, such as a valve 60 and 62 respectively.

Tank 56 associated with lines 54 and 58, and with valves 60 and 62,provides reversibility of the heat exchange from the evaporator so as toturn the cooler function of this evaporator into a heat generatorfunction for heating the exhaust gas circulating in portion 40.

When the engine is running, the calculator (associated with detector 31)can evaluate that the exhaust gas temperature is either excessive (aboveabout 500° C.) or insufficient (below about 300° C.) to provide smoothrunning of SCR catalyst 30.

In cases where this temperature is excessive, circuit 34 is started soas to cool the exhaust gas circulating in portion 40 of line 20 whileproviding heat exchange between the gas circulating in this portion andevaporator 48.

More precisely, valves 60, 62 are in closed position for lines 54, 58and the water circulates in this circuit in a conventional clockwisedirection with respect to the figure (arrows A) under the effect ofcompressor 36. The compressed water leaving this compressor circulatesin line 46 and ends into evaporator 38. This compressed water then flowsthrough the evaporator by collecting the heat carried by the exhaustgas, which is transmitted to this evaporator. Under the effect of thisheat from the gas, the temperature thereof is lowered and the water isheated, thus leaving the evaporator in form of hot compressed steam. Thesteam then flows through expander 42 while transmitting thereto theenergy it contains. The expanded steam leaving this expander throughline 50 flows through condenser 44, which it leaves in form of liquidwater. This liquid water is finally brought through line 52 tocompressor 36 in order to be compressed.

Circuit 10 is thus kept in operation until the temperature of theexhaust gas reaching the inlet face of the SCR catalyst is thetemperature required for operation of this catalyst.

Just before the circuit is stopped, valve 60 of connecting line 58 isswitched to an open position so that the steam leaving evaporator 38 isfed into thermally insulated storage tank 56 where it is kept at hightemperature.

This valve is then set to a closed position as soon as the tank isfilled with this steam.

Conversely, in case of an insufficient exhaust gas temperature foroperation of SCR catalyst 30, notably when starting the engine, valves60 and 62 are set to an open position for lines 58, 54 and compressor 36is started.

In this configuration, the high-temperature steam contained in tank 56is discharged therefrom through connecting line 58 and fed into line 48.Under the effect of compressor 36, this steam circulates clockwise withrespect to arrows A′ in FIG. 1.

The steam thus circulates in a portion of line 48, then in bypass line54 before it reaches the compressor inlet. This steam leaves thecompressor and enters evaporator 38. This evaporator thus inverts itsinitial function of collecting the heat contained in the gas. Moreprecisely, the evaporator turns into a heat generator by yielding theheat contained in the steam to the exhaust gas circulating in portion 40of the exhaust line. This allows the gas to be heated by thermalexchange. It is therefore possible to rapidly increase the gastemperature and to considerably reduce the time required to obtain thesuitable exhaust gas temperature for operation of the SCR catalyst.

Advantageously, an additional valve 64 (in dotted line in FIG. 1) can beprovided on line 50 or 52 so that, in closed position for these lines,the steam cannot flow through condenser 44.

Of course, this additional valve is in an open position for lines 50 or52 in the configuration where this circuit is used for cooling theexhaust gas.

Thus, by means of simple layouts of this closed circuit, it is possibleto either cool the exhaust gas or to reversibly heat it using the sameevaporator.

Of course, without departing from the scope of the invention, it ispossible to use, instead of the thermally insulated tank, a tankprovided with means for heating the liquid it contains, such as heatingresistors, a burner, etc.

FIG. 3 shows a variant of the reversible heat exchange device 32provided between the two catalysts 24, 30 to control the temperature ofthe exhaust gas reaching inlet face 28 of SCR catalyst 30.

This device comprises a thermopile or a succession of thermopiles 66arranged on portion 40 of exhaust line 20 between three-way catalyst 24and SCR catalyst 30.

Generally, this thermopile allows to recover the heat energy containedin the exhaust gas and to convert it, notably through Seebeck effect, toan electric energy that is then stored in electric accumulators 68through conductors 70.

Of course, as in FIG. 1, the person skilled in the art can consider allthe possible configurations for arranging this thermopile in direct orindirect connection with this exhaust line portion so as to provide thebest possible heat exchange with the exhaust gas.

Thus, in cases where the exhaust gas temperature is excessive (aboveabout 500° C.), the thermopile is active to provide heat exchangebetween the gas circulating in portion 40 of line 20 and thisthermopile, by collecting the heat contained in this gas. The exhaustgas is therefore cooled and the thermopile collects the heat energycontained in the exhaust gas stream so as to convert it to electricenergy that is stored in accumulators 68.

Once the exhaust gas temperature is stabilized, operation of thethermopile can be stopped.

In the opposite case where the exhaust gas temperature is below itsminimum threshold value required to provide operation of SCR catalyst30, thermopile 66 is supplied with power by accumulators 68.

This power supply has the effect of heating the thermopile that can thentransfer its heat to the exhaust gas circulating in portion 40 of line20.

Similarly, once the exhaust gas temperature has reached the desiredvalue, the thermopile supply is stopped so as to stop heating theexhaust gas, which has become unnecessary.

Thanks to the reversibility of the thermopile heat exchange, the exhaustgas can be cooled or heated so that its temperature is in the usualoperating range of the SCR catalyst.

1) A method for controlling the temperature of exhaust gas circulatingin an exhaust line of an internal-combustion engine, said linecomprising means of treating pollutants contained in this gas and heatexchange means for cooling or heating said exhaust gas flowing throughthese pollutant treatment means, characterized in that it consists inusing evaporator of a fluid circulation closed circuit as heat exchangemeans. 2) A method as claimed in claim 1, characterized in that itconsists in feeding a hot fluid into the evaporator so that it heats theexhaust gas. 3) A method as claimed in claim 2, characterized in that itconsists in using a hot fluid contained in a fluid storage meansconnected to the circuit. 4) A method as claimed in claim 1,characterized in that it consists in connecting storage means toevaporator by means of a bypass line. 5) A device for controlling thetemperature of exhaust gas circulating in an exhaust line of aninternal-combustion engine, said line comprising means of treatingpollutants contained in this gas and reversible heat exchange means forcooling or heating said exhaust gas flowing through these pollutanttreatment means, characterized in that the reversible heat exchangemeans are an evaporator of a fluid circulation closed circuit. 6) Adevice as claimed in claim 5, characterized in that circuit comprises astorage tank for a hot fluid. 7) A device as claimed in claim 6,characterized in that the tank is a thermally insulated tank. 8) Adevice as claimed in claim 6, characterized in that the tank comprisesfluid heating means. 9) A device as claimed in claim 5, characterized inthat circuit comprises a bypass line for allowing the fluid from thetank into evaporator. 10) A device as claimed in claim 9, characterizedin that bypass line carries a throttling means. 11) A device as claimedin claim 5, characterized in that the pollutant treatment means comprisea selective catalytic reduction catalyst.